This invention relates to a power supply for driving a He-Ne laser.
A known power supply of the type comprises a DC power source for generating a DC power with a constant voltage. A switching element is connected to the DC power source and switches the DC power to produce a pulse power having a pulsive voltage waveform repeated with a controllable duty ratio. A driver circuit is connected to the switching element and drives the switching element to make the controllable duty ratio into a predetermined ratio. The driver circuit is responsive to a driver control signal and drives the switching circuit to make the controllable duty ratio into a predetermined reduced duty ratio to reduce the pulse power. A high-voltage generating circuit is connected to the DC power source and the switching circuit and generates, as a high-voltage DC power, a DC power of a high voltage corresponding to the pulse power. An output port is coupled to the high-voltage generating circuit and is for connection of the He-Ne laser tube. The high-voltage DC power is supplied to and drives the He-Ne laser tube connected to the output port. The switching element, the driver circuit, and the high voltage generating circuit form a, so called, DC-DC converter for generating a high voltage DC power from a DC power of a relatively low voltage from the DC power source.
The known power supply further comprises a detector connected between the high-voltage generating circuit and the output port. The detector detects a DC electric current flowing through the He-Ne laser tube connected to the output port to produce a detected voltage signal having a detected voltage level corresponding to the DC electric current. A feedback circuit is connected to the detector and compares the detected voltage signal with a feedback reference voltage having a feedback reference voltage level to produce the driver control signal when the detected voltage level is larger than the feedback reference voltage level.
In the known power supply, the DC current flowing the He-Ne laser is stabilized by the feedback control. That is, when the DC current is increased above a predetermined level corresponding to the feedback reference voltage level, the drive control signal is produced from the feedback circuit, and the pulse power reduced is supplied to the high-voltage generating circuit, so that the high-voltage DC power is reduced in the voltage level. Accordingly, the DC current decreases and becomes the predetermined level or below, the control signal is then stopped so that the pulse power is increased to increase the DC current flowing through the He-Ne laser. Thus, the DC current flowing through the He-Ne laser is stablized constant at the predetermined level.
Discharge or operation of the He-Ne laser is stably maintained by supply of DC power of a high voltage such as several kilo volts after the discharge starts. However, the higher voltage is required to start the discharge. Accordingly, the high-voltage generating circuit in the conventional power supply is provided with two high DC voltage generating circuits, first one for starting the discharge and second one for maintaining the discharge after start. In operation, the higher voltage is at first supplied from the first high DC voltage generating circuit to the He-Ne laser to start discharge. After start of the discharge, the He-Ne laser is lowered in its impedance so that the first high DC voltage generating circuit stops operation and the second high DC voltage generating circuit supplies the high voltage to the He-Ne laser in place of the first high DC voltage generating circuit.
At driving start of the He-Ne laser by the power supply, the higher voltage is applied to the He-Ne laser so as to start discharge of the laser and DC current flowing through the laser is therefore higher than the predetermined level. That is, a, so called, rush current flows through the laser and the detector. Accordingly, the driver control signal is produced by the feedback circuit and the pulse power is therefore reduced so that the DC current suffers from undershoot. That is, the DC current lowers below the predetermined level just after the start of the discharge, so that the discharge unfortunately is stopped.